1. Field of the Invention
The invention relates to a receiver, and particularly to a receiver suitable for the OFDM system employed in a mobile communications system, a wireless LAN system, and the like.
2. Description of the Related Art
In recent years, digital television broadcasting has been started. In Europe and Japan, the orthogonal frequency division multiplexing (OFDM) modulation system is employed as the transmission system of the terrestrial television broadcasting system. In the OFDM system, wideband signals are transmitted by use of multiple subcarriers that are orthogonal to one another. For this reason, the OFDM system is advantageous in that the OFDM system has, as one of essential transmission conditions for the terrestrial television broadcasting, an ability to improve delay interference characteristics in the multipath propagation paths.
The digital broadcasting in Japan employs the ISDB-T standard. According to the ISDB-T standard, a transport stream (TS) specified in the standard of Moving Picture Experts Group (MPEG) undergoes various signal processings such as error-correcting coding, interleave coding, and digital modulation. The resultant signals are OFDM modulated, and are then outputted.
According to the ISDB-T standard, a single block includes OFDM symbols of 108 carriers in the frequency domain. A single segment includes a single block, two blocks, or four blocks, depending on the mode. Accordingly, the number of carriers for each single segment is 108, 216, or 432. According to the ISDB-T standard, the transmission of signals is performed using a bandwidth for 13 segments.
In addition, according to the ISDB-T standard, a single frame includes 204 OFDM symbols in the time domain. The TS-transmission and the energy spread processing are performed frame by frame.
In addition, the ISDB-T standard can provide a hierarchical transmission, in which plural layers of different transmission characteristics are transmitted simultaneously. Each layer includes one or plural OFDM segments. Parameters, such as the carrier modulation system, the coding ratio of inner coding, and the time interleave length, can be specified for each layer.
In digital broadcasting receivers, TSs are extracted from the broadcasting signals of a channel selected by the tuner, through processing on the broadcasting signals by various processing units including an orthogonal detector unit, a FFT unit, a demodulator unit, and a decoder unit. For example, Japanese Patent Application Publication No. 2003-273824 proposes an apparatus which supplies TSs from a decoder unit to a TS decoder by selectively outputting TSs of a layer that is determined as providing effective output. This selective TS-output allows the reduction in power consumption as well as saves a space of the recording area in the recording medium when the broadcasting is recorded in a subsequent stage.
The OFDM receiving demodulator unit including the orthogonal detector unit, the FFT unit, the demodulator unit, and the decoder unit is often formed in an LSI circuit. In addition, the OFDM receiving demodulator unit and the tuner are included together in an LSI circuit in some cases nowadays.
In general, the internal voltages of LSI circuits tend to become lower to reduce the power consumption. Nonetheless, the voltage of the TS-output pin is usually set at a higher voltage than the internal voltage so that the TS-output pin maintains the interfaces with other LSI circuits. Accordingly, the amplification factor at the TS-output portion in the OFDM receiving demodulator unit is so large that spurious (high-frequency noise) emissions are more likely to occur. In addition, the technical progress achieved in the LSI circuit development process has made the chip size smaller. In a smaller-sized chip, the distance between each adjacent pins of the LSI circuit is narrower, and so is the distance between the tuner input pin and the TS-output pin.
For this reason, spurious emissions from the TS-output pin are likely to be inputted into the tuner input pin. The input of the spurious emissions causes no problems when the broadcasting signals are received. The input, however, causes a problem while no signal is received because synchronous detections are performed for the spurious-components mixed in through the tuner input pin, and thus synchronization is secured in the OFDM receiving demodulator unit. Upon securing the synchronization, the OFDM receiving demodulator unit sometimes starts operating other portions inside. The erroneous operations result in wasteful consumption of power. In addition, securing synchronization has another problem that circuits disposed at stages subsequent to the OFDM receiving demodulator unit, such as a decoder, may operate erroneously as well, so that power is wastefully consumed.